Ignition system for an internal combustion engine

ABSTRACT

An ignition system for an internal combustion engine having an AC generator comprises an ignition coil energized by an AC output from the generator. A first thyristor is connected in series with the primary of the ignition coil and permits and interrupts the current through the primary. The first thyristor is turned on at a first angle in advance of ignition. A capacitor is charged by an AC output from the generator, and is discharged when a second thyristor conducts to apply a reverse voltage across the anode and cathode of the first thyristor. The second thyristor is triggered at a second angle.

BACKGROUND OF THE INVENTION

The present invention relates to an ignition system for an internalcombustion engine.

A conventional ignition system as is disclosed in U.S. Pat. No.3,424,944 filed Nov. 9, 1966 comprises a first thyristor connected inseries with the primary winding of an ignition coil, a second thyristorhaving the cathode thereof connected to the cathode of the firstthyristor and a capacitor coupling the anodes of the first and secondthyristors. A DC power source supplies a DC current at a constantvoltage. The first thyristor is turned on in advance of the ignitiontiming of the engine. The current through the primary winding isestablished. Meantime, the capacitor is charged through a path includingthe first thyristor. At the ignition timing, the second thyristor isturned on and the capacitor is discharged through the second thyristorto apply a reverse voltage across the anode and cathode of the firstthyristor, so that the first thyristor is turned off. The turn-off ofthe first thyristor leads to sudden decrease of the primary current,which in turn causes generation of a high voltage in the secondarywinding.

It is first noted that this conventional ignition system is intended tobe used in an engine provided with a DC power source of constant voltagesuch as batteries and does not operate in an engine which is providedwith an AC generator instead of batteries.

Secondly, as the output voltage from the DC power source is constant thevoltage applied to the primary winding is constant regardless of theengine speed. Inductance and resistance of the primary winding and ofany element in series circuit with the primary winding as well as thevoltage applied to the primary winding determine the length of timerequired for the primary current to be established, or in other words,to reach a value sufficient to cause, when suddenly reduced to nil, ahigh voltage in the secondary winding. Since the inductance, theresistance and the voltage applied are all constant in this conventionalsystem, the time required for establishing the primary current isconstant regardless of the engine speed. On the other hand, the timerequired for one cycle of operation becomes shorter as the engine speedincreases. As a result, at engine speeds which are too high for theprimary current to be fully established, a voltage generated in thesecondary winding is insufficient. This means that the engine speed maybe limited by the factors of the resistance and the inductance as wellas the voltage.

Another factor that may limit the engine speed is the time required forcharging the capacitor. The capacitor has to be charged to a voltagewhich is sufficient to turn off the first thyristor when the capacitoris subsequently discharged through the second thyristor to apply areverse voltage across the anode and cathode of the first thyristor. Inthis conventional system, the capacitor is charged by an oscillator andthis charging begins when the first thyristor conducts. Output voltageof the oscillator is constant and therefore the time required forcompleting the charging is constant regardless of the engine speed. Asthe engine speed becomes so high that time allotted for the fullcharging of the capacitor is insufficient, the first thyristor is notturned off by the reverse voltage from the capacitor which is notsufficient with the result that the primary current is not interrupted,and ignition of the engine does not take place. Consequently, this maylimit the engine speed.

Also as the ignition angle advances with increasing engine speed, thetime allowed for establishing the primary current and for charging thecapacitor becomes shorter because not only of the decreasing period ofone cycle which is inversely proportional to the engine speed, but alsoof advancing engine speed where some means for automatically advancingthe ignition angle is incorporated as is often desired. This may furtherlimit the engine speed.

Summary of the Invention

An object of the present invention is to provide an ignition system foran internal combustion engine having an AC generator rotating insynchronism with the engine and particularly an ignition system in whicha current through the primary winding of an ignition coil is suddenlychanged, without failure at various engine speeds, to induce a highvoltage in the secondary winding.

Another object of the present invention is to provide an ignition systemin which the voltage applied to the primary winding of an ignition coilincreases as the engine speed increases, and therefore the currentthrough the primary winding is established within a period of time whichbecomes shorter with increasing enging speed, so as not to set an upperlimit to the engine speed.

Another object of the present invention is to provide an ignition systemin which the voltage used for charging the capacitor increases as theengine speed increases, and therefore the charging is completed within aperiod of time which becomes shorter with increasing engine speed, so asnot to limit the engine speed.

Another object of the present invenion is to enhance the spark energy atthe ignition plug in an ignition system by superimposing a spark due toa capacitor discharge and a spark due to interruption of the primarycurrent.

Still another object of the invention is to provide an ignition systemin which the timing or angle at which ignition occurs advances as theengine speed increases.

An ignition system of the present invention comprises an ignition coilhaving a primary winding and a secondary winding. An ignition plug isconnected across the secondary winding. The ignition system furthercomprises a first thyristor connected in series with the primary windingto control a current through the primary winding. In an embodiment ofthe present invention, the primary winding of the ignition coil isenergized by a first armature winding disposed in the AC generator. Inanother embodiment, the ignition coil is disposed in the generator togenerate electricity for itself.

The ignition system further comprises a second thyristor and a secondarmature winding disposed in the AC generator. A capacitor is connectedthrough a diode across the output terminals of the second armaturewinding. A first signal source is adapted to supply a first triggersignal to the gate of said first thyristor at a first angle in advanceof the ignition timing or angle of the engine. A second thyristor sourceis also adapted to supply a second trigger signal to the gate of thesecond thyristor at a second angle. The first and second trigger signalmay respectively be produced at fixed first and second angles, or mayalternatively advance with increasing engine speed.

The capacitor is further connected to apply a reverse voltage across theanode and cathode of the first thyristor when the second thyristorconducts, so that the first thyristor is turned off. Turn-off of thefirst thyristor and therefore interruption of the current through theprimary winding induces a high voltage in the secondary winding.

In an embodiment of the invention, when the second thyristor conducts adischarge current is made to flow through the primary winding. Thedischarge current induces a high voltage in the secondary winding priorto the generation of a high voltage in the secondary winding due to theinterruption of the current through the primary winding.

Where a first and second signal sources supply a first and secondtrigger signals at a first and second angles which do not automaticallyadvance with increasing engine speed but are fixed, it is desirable toincorporate an arrangement for effecting automatic advance in theignition circuit other then the signal sources themselves. A particularembodiment of the present invention has been devised for this purpose,in which a resistor is connected in series with the first thyristor andboth ends of the resistor are connected through a diode across the gateand cathode of the second thyristor. The voltage across the resistorchanges in proportion to the current through the primary winding. Thecurrent through the primary begins to flow when the first thyristor isturned on. The peak value of the current as well as the rate of riseincreases as the engine speed increases. The voltage across the resistordoes not reach the trigger level of the second thyristor while theengine speed is low, and the second thyristor is turned on when thesecond signal source supplies a trigger signal to the second thyristor.At engine speeds exeeding a certain value, the voltage across theresistor reaches the trigger level before the second signal sourcesupplies the trigger signal. The higher the engine speed, the higher therate of rise of the voltage across the resistor and therefore theearlier the trigger of the second thyristor. The result is that ignitiontiming or angle advances as the engine speed increases.

According to the present invention, the voltage applied to the primarywinding of an ignition coil is generated in winding disposed in an ACgenerator rotating in synchronism with the engine. The voltage thereforeincreases as the speed of the engine increases. Accordingly, the timerequired for the current through the primary winding to be fullyestablished becomes shorter as the engine speed increases, and therebydoes not limit the engine speed.

Also, the voltage used for charging the capacitor is derived from awinding disposed in the AC generator, and increases with the enginespeed. Accordingly, the time required for completing the charging of thecapacitor becomes shorter as the engine speed increases and thereby doesnot limit the engine speed.

In some of the embodiments of the present invention, a discharge currentfrom the capacitor is made to flow through the primary winding of anignition coil to induce a high voltage in the secondary winding toprovide additional spark energy prior to the high voltage generation dueto the interruption of the primary current. Spark energy is therebyenhanced.

Moreover, some embodiments of the present invention employ anarrangement for automatically and electrically advancing ignition anglewith increasing engine speed, the arrangement being included in theignition system other than the signal sources.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 shows a circuit diagram of an embodiment of an ignition systemaccording to the present invention,

FIG. 2A is a sectional front view of a flywheel magneto generatorprovided with signal generators,

FIG. 2B is an elevational view of a first and second armatures disposedin the flywheel magneto generator of FIG. 2A,

FIG. 3A through 3H show waveforms of voltage or current at variousportions of the ignition system according to the present invention,

FIGS. 4 and 5 show circuit diagrams of other embodiments of the presentinvention,

FIG. 6 shows a waveform of the voltage across the secondary winding ofthe ignition coil of the embodiment of FIG. 5,

FIGS. 7 and 8 show circuit diagrams of other embodiments of the presentinvention,

FIGS. 9A and 9B respectively show a waveform of the voltage across aresistor connected in series with a first thyristor and a second triggersignal,

FIG. 10 shows advance of ignition angle plotted against engine speed,

FIGS. 11 through 13 show circuit diagrams of further embodiments of thepresent invention.

Detailed Description of the Preferred Embodiments

Referring now more particularly to FIG. 1, there is shown an embodimentof the ignition system according to the present invention. Designated bynumeral 1 is an AC generator, such as a flywheel magneto generator,which rotates in synchronism with an internal combustion engine.Generator 1 is provided with a first armature winding 2 and a secondarmature winding 3. Armature windings 2 and 3 produce an AC output ofthe same phase in this embodiment. An ignition coil 4 has a primarywinding 4a and a secondary winding 4b. Primary winding 4a is connectedthrough a first thyristor 5 across the output terminals of firstarmature winding 2. An ignition plug 6 is connected across secondarywinding 4b. A capacitor 7 is connected through a second thyristor 8across the anode and cathode of first thyristor 5. More particularly,capacitor 7 is connected between the anodes of thyristors 5 and 8, andthe cathodes of thyristors 5 and 8 are directly connected to each other.Capacitor 7 has both terminals connected through a diode 9 across theoutput terminals of second armature winding 3. Capacitor 7 is chargedinto a polarity as indicated by marks "+" and "-" in FIG. 1 during ahalf cycle when second armature winding 3 produces an output of apolarity opposite to that indicated in FIG. 1.

A first signal source 10 is connected to supply a first trigger signalto the gate of first thyristor 5 at a first angle as the engine rotates.A second signal source 11 is connected to supply a second trigger signalto the gate of second thyristor 8 at a second angle as the rotation ofthe engine proceeds.

Each of signal sources 10 and 11 may for instance comprise a signalgenerating coil mounted on a substantially I-shaped armature core or onone of radially outwardly extending portions of a substantiallystar-shaped core disposed inside an AC generator rotating in synchronismwith the engine to cooperate with the magnets of the generator, andmeans for producing a trigger pulse at a rotational angle when theoutput voltage of the signal generating coil reaches a predeterminedvalue. In this case, the angle at which the output voltage reaches apredetermined value more or less advances as the engine speed increasessince the peak value as well as the rate of rise of the output signalincreases with engine seed. Automatic advance of ignition angle is atleast to some extent effected without resorting to any furtherarrangement.

Alternatively, where a flywheel magneto generator, such as shown inFIGS. 2A and 2B is employed, each of signal sources 10 and 11 maycomprise a signal generator 31 or 32 which is positioned outside andclose to the flywheel 33 of the flywheel magneto generator and producesa trigger pulse when a portion 34 extending from a pole piece 35 on oneof the magnets 36 mounted inside flywheel 33 through an opening 37provided in flywheel 33 approaches and passes opposite to signalgenerator 31 or 32 as flywheel 33 rotates. In this case, trigger pulsesare produced at substantially fixed angles regardless of the enginespeed, so that some means for advancing the ignition angle has to beadded where advance of ignition angle is desired.

In either case, signal sources 10 and 11 are so designed, or the signalgenerating coils are so positioned that a first trigger signal isproduced nearly at the beginning of a half cycle when first armaturewinding 2 produces an output of a polarity as indicated in FIG. 1 and asecond trigger signal is produced at an angle when the current throughthe primary winding 4a of ignition coil 4 is nearly at its peak value.

FIG. 2A together with FIG. 2B also show, by way of example, how firstand second armature windings 1 and 2 may be mounted whereabove-mentioned flywheel magneto generator is employed. Designated bynumeral 38 is a fixed plate to which first and second armature windings1 and 2 wounded on substantially I-shaped cores are fixed. A generatingwinding 39 which energizes head lights and the like is also mounted,spaced from armature windings 1 and 2 by 180°.

Operation of the above embodiment is now described with reference toFIGS. 3A through 3G, which show waveforms of voltage or current atvarious portions of the system against rotational angle θ ofgenerator 1. FIGS. 3A, 3B and 3C respectively show variations of amagnetic flux φ, a no-load voltage V₁ of first armature winding 2 and ano-load voltage V₂ of second armature winding 3. FIGS. 3D and 3Erespectively show a first and second trigger signals S₁ and S₂ producedby first and second signal sources 10 and 11. FIGS. 3F and 3Grespectively show the current I₁ through primary winding 4a and voltageVc across capacitor 7. FIG. 3H will be referred to later.

As already mentioned, generator 1 rotates in synchronism with theengine. During a half cycle when second armature winding 3 produces anoutput of a polarity opposite to that shown in FIG. 1, capacitor 7 ischarged into a polarity as shown in FIG. 1 by the output of secondarmature winding 3 through diode 9. During the next half cycle, whenfirst armature winding 2 produces an output of a polarity as shown inFIG. 1, first signal source 10 supplies a first trigger signal S₁ to thegate of first thyristor 5 at a first angle θ₁ close to the beginning ofthe half cycle, so that first thyristor 5 is turned on and the currentI₁ through the primary winding begins to flow.

Thereafter, at a second angle θ₂ at which the primary current I₁ isapproximately at its peak value, second signal source 11 supplies asecond trigger signal S₂ to the gate of second thyristor 8. Since secondthyristor 8 is forward-biased by the voltage across capacitor 7, secondthyristor 8 becomes conductive upon receipt of second trigger signal S₂.As second thyristor 8 conducts, the voltage built up on capacitor 7 isapplied in a reverse direction across the anode and cathode of firstthyristor 5, so that first thyristor 5 is turned off. Capacitor 7 isdischarged through a path including second thyristor 8, first armaturewinding 2 and primary winding 4a of ignition coil 4, and is subsequentlycharged into a polarity opposite to that shown in FIG. 1. When thecharging of capacitor 7 into such opposite polarity is completed, secondthyristor 8 turns off. The result is that the current through theprimary winding 4a drops to nil within a time as short as a few tens ofmicrosecond, so that a high voltage is induced in the second winding 4band thereby plug 6 is fired. Thus, by setting the timing of occurrenceof second trigger signal S₂ at the ignition angle (precisely speaking, alittle before it allowing for the time required for the discharge andsubsequent charge of capacitor 7), ignition of the engine isappropriately timed.

According to the embodiment described above, first armature winding 2disposed in AC generator 1 rotating in synchronism with the enginesupplies a current to primary winding 4a, so that the current isestablished within a period of time which becomes shorter withincreasing engine speed. Accordingly, the engine speed is not limited bysuch time required for establishing the primary current. Also, capacitor7 is charged by an output from second armature winding 3 disposed in ACgenerator 1 within a period of time which becomes shorter withincreasing engine speed, so that the engine speed is not limited by suchtime required for charging capacitor 7.

FIG. 4 shows another embodiment of an ignition system according to thepresent invention. In this embodiment, the anodes of first and secondthyristors 5 and 8 are connected to each other and capacitor 7 isconnected between the cathodes of thyristors 5 and 8. The rest ofconnections and the operation of the embodiment are substantiallyidentical to those described about the embodiment of FIG. 1.

FIG. 5 shows still another embodiment of the present invention.Connections of this embodiment is substantially identical to those inFIG. 1 except that a diode 12 is connected across a series circuitformed of primary winding 4a of ignition coil 4, capacitor 7 and secondthyristor 8. More particularly, the anode of diode 12 is connected tothe cathode of thyristor 8 and the cathode of diode 12 is connected toan end of primary winding 4a. A diode 13 is inserted between an end offirst armature winding 2 and the cathode of diode 12 to prevent ashort-circuit current to flow during half cycles when an output voltageof first armature 2 is in a polarity opposite to that shown in FIG. 5.

Effect of the incorporation of diode 12 is that a discharge current fromcapacitor 7 upon conduction of thyristor 8 flows through secondthyristor 8, diode 12 and primary winding 4a of ignition coil 4. Becausethis discharge current flows through a path excluding first armaturewinding 2, the peak value of the current and the rate of rise of thecurrent is, as shown in FIG. 3H, larger than those obtained in the caseof the embodiment of FIG. 1. Consequently, a high voltage is generatedin secondary winding 4b of ignition coil 4 when the discharge currentflows through primary winding 4a, prior to generation of a high voltagedue to subsequent interruption of the primary current. Polarity of thesecondary voltage indicated by E₁ in FIG. 6 due to the rise of thedischarge current is opposite to that of the secondary voltage indicatedby E₂ due to the drop or interruption of the current. Accordingly, plug6 sparks in one polarity and then in the opposite polarity. The totalspark energy is larger than that obtained where only a spark due to theinterruption of the primary current is utilized. As is known in the art,enhancement of the spark energy is one of the desiderata in view ofprevention of environmental polution.

FIG. 7 shows another embodiment of the present invention. Connections ofthis embodiment are substantially identical to those in FIG. 5 exceptthat the anodes of first and second thyristors 5 and 8 are connected toeach other and capacitor 7 is connected between the cathodes ofthyristors 5 and 8. The operation of this embodiment is substantiallyidentical to that of the embodiment of FIG. 5.

The various embodiments described hereinbefore either employ signalsources which supply trigger signals at fixed angles, or alternativelyrely on signal sources whose output trigger signals are produced at moreadvanced angles with increasing engine speed. The latter signal sources,such as those disposed inside an AC generator, are not always available,because the space inside an AC generator is limited.

It is therefore desired that an arrangement for automatically advancingthe ignition angle and for preventing failure of ignition at low enginespeeds by provided in an ignition circuit other than the signal sourcesthemselves.

FIG. 8 shows an embodiment of the present invention satisfying suchrequirement. Connections of this embodiment are substantially identicalto those in FIG. 5 except that a resistor 14 is connected in series withfirst thyristor 5 and both ends of resistor 14 is connected through adiode 15 across the gate and cathode of second thyristor 8, so that thevoltage across resistor 14 is applied to the gate of second thyristor 8.

As described earlier, the output voltage from first armature winding 2increases with the engine speed. Accordingly, the voltage Vr acrossresistor 14 which is proportional to the current through the primarywinding 4a and through resistor 14 itself changes taking waveforms asshown in FIG. 9A for various engine speed Na, Nb, Nc and Nd(Na<Nb<Nc<Nd), supposing first thyristor 5 is turned on at angle θ₁ andis not turned off until the current through the primary windingnaturally falls to nil.

While the engine sped is at Na, voltage Vr is slow to rise and is notlarge enough to reach the trigger level Vt of second thyristor 8. So,second thyristor 8 is turned on upon receipt of second trigger signal S₂as shown in FIG. 9B from second signal source 11 at angle θ₂. As theengine speed increases, voltage Vr become rapid to rise and becomeslarger, and therefore becomes to reach trigger level Vo, at angle θb forengine speed Nb, at θc for Nc and at θd for Nd.

Thus, the ignition system can be so designed that as the engine speedexceeds a predetermined value second thyristor 8 is triggered by voltageVr cross resistor 14 before second signal source 11 supplies secondtrigger signal S₂, and thereby the angle at which ignition takes placeadvances with increasing engine speed, while at speeds below thepredetermined value second thyristor 8 is turned on by second triggersignal S₂ at a fixed angle θ₂. Consequently, angle of advance varieswith engine speed N as shown in FIG. 10.

The embodiments described hereinbefore include an ignition coil and aseparate first armature winding. However, it is also possible to disposethe primary winding of an ignition coil in an AC generator and have itgenerate electricity for itself as, for example, is shown in FIG. 11. Inthis embodiment, an ignition system comprises an ignition coil 16 havinga primary winding 16a and a secondary winding 16b. Primary winding 16ais disposed in generator 1. Secondary winding 16b is magneticallycoupled to primary winding 16a. A second armature winding 17 similar tosecond armature winding 3 in FIG. 1 is also disposed in generator 1 toprovide an output to charge capacitor 7 through diode 9. First thyristor5 is connected to the ends of primary winding 16a of ignition coil 16.The rest of connections are substantially identical to those in FIG. 1.

When first thyristor conducts a short-circuit current flows throughthyristor 5. When second thyristor 8 conducts capacitor 7 which has beencharged by second armature winding 17 is discharged through primarywinding 16a to induce a high voltage in secondary winding 16b. Also,when capacitor 7 is discharged first thyristor 5 is turned off, leadingupon completion of the discharge to interruption of the current throughprimary winding 16a and therefore generation of a high voltage insecondary winding 16b.

FIG. 12 shows a further embodiment of the present invention. Connectionsof this embodiment are substantially identical to those in FIG. 11except that the anodes of first and second thyristors 5 and 8 areconnected to each other and capacitor 7 is connected between thecathodes of thyristors 5 and 8. The operation of this embodiment issubstantially identical to that of the embodiment of FIG. 11.

FIG. 13 shows a still further embodiment of the present invention.Connections of this embodiment are substantially identical to those inFIG. 11 except that a resistor 14 is connected in series with firstthyristor 5 and both ends of resistor 14 is connected through a diodeacross the gate and cathode of second thyristor 8, so that the voltageacross resistor 14 is applied to the gate of second thyristor 8.

As will be apparent from the description made with reference to FIG. 8,second thyristor 8 is triggered by the voltage across resistor 14 beforesecond signal source 11 supplies second trigger signal S₂ as the enginespeed exceeds a predetermined valve, while at speeds below thepredetermined value second thyristor 8 is turned on by second triggersignal S₂ at fixed angle θ₂, thereby enabling advance of ignition angle.

The sequence of operation after second thyristor 8 is turned on aresubstantially identical to that of the embodiment of FIG. 11.

Although capacitor 7 has been described as being charged during a halfcycle when the polarity of the output voltage from the second armatureis opposite to that shown in the various circuit diagrams, it is alsopossible to connect the capacitor so as to achieve the charging during ahalf cycle when the polarity of the output voltage from the secondarmature winding is as shown in the circuit diagrams.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed,therefore, in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. An ignition system for an internal combustionengine having an AC generator rotating in synchronism with the engine,said ignition system comprising; an ignition coil having a primarywinding and a secondary winding, an ignition plug connected across saidsecondary winding of said ignition coil, a first armature windingdisposed in the generator to supply a current to said primary winding ofsaid ignition coil, a first thyristor connected in series with saidprimary winding of said ignition coil to control the current throughsaid primary winding, a second thyristor, a second armature windingdisposed in the generator, a capacitor connected through a diode acrosssaid second armature winding, a first signal source to supply a firsttrigger signal to the gate of said first thyristor at a first angle, anda second signal source to supply a second trigger signal to the gate ofsaid second thyristor at a second angle, said capacitor being furtherconnected to apply a reverse voltage across the anode and cathode ofsaid first thyristor when said second thyristor conducts, so that saidfirst thyristor is turned off, whereby interruption of the currentthrough said primary winding of said ignition coil induces a high volagein said secondary winding of said ignition coil.
 2. An ignition systemas set forth in claim 1, wherein said primary winding of said ignitioncoil is connected through said first thyristor to the output terminalsof said first armature winding and said capacitor is connected throughsaid second thyristor across the anode and cathode of said firstthyristor.
 3. An ignition system as set forth in claim 2, furthercomprising a diode across a series circuit formed of said primarywinding of said ignition coil, said capacitor and said second thyristorto allow a discharge current of said capacitor to flow through a pathincluding said diode and said primary winding, whereby the dischargecurrent through said primary winding of said ignition coil induces ahigh voltage in said secondary winding prior to the induction of a highvoltage due to the interruption of the current through said primarywinding.
 4. An ignition system as set forth in claim 1, furthercomprising a resisitor connected in series with said first thyristor,both ends of said resistor being connected through a diode across thegate and cathode of said second thyristor, wherein as the rotationalspeed of the engine exceeds a predetermined value the voltage acrosssaid resistor reaches the trigger level of said second thyristor beforesaid second signal source supplies the second trigger signal to saidsecond thyristor.
 5. An ignition system for an internal combustionengine having an AC generator rotating in synchronism with the engine,said ignition system comprising; an ignition coil having a primarywinding and a secondary winding, said primary winding being disposed inthe generator to provide a current and said secondary winding beingmagnetically coupled to said primary winding, an ignition plug connectedacross said secondary winding of said ignition coil, a first thyristorhaving the anode and cathode connected across said primary winding ofsaid ignition coil, a second thyristor, a second armature windingdisposed in the generator, a capacitor connected through a diode acrosssaid second armature winding, a first signal source to supply a firsttrigger signal to the gate of said first thyristor at a first angle, asecond signal source to supply a second trigger signal to the gate ofsaid second thyristor at a second angle, said capacitor being furtherconnected through said second thyristor across the anode and cathode ofsaid first thyristor to apply a reverse voltage across the anode andcathode of said first thyristor when said second thyristor conducts, sothat said first thyristor is turned off.
 6. An ignition system as setforth in claim 5, further comprising a resistor connected in series withsaid first thyristor, both ends of said resistor being connected througha diode across the gate and cathode of said second thyristor, wherein asthe rotational speed of the engine exceeds a predetermined value thevoltage across said resistor reaches the trigger level of said secondthyristor before said second signal source supplies the second triggersignal to said second thyristor.